The invention relates to a cathode ray tube comprising means for generating an electron beam, means for deflecting the electron beam, and a coil for influencing a deflection of the electron beam.
WO 99/34392 (attorneys"" docket PHN 16.716) describes a color display device comprising a cathode ray tube including an electron gun for generating three electron beams, a color selection electrode, deflecting means for deflecting the electron beams located at a deflection plane, and first and second influencing means to dynamically influence the convergence of the electron beams, to decrease a distance between the electron beams at a location of the deflection plane. The deflecting means comprise a set of line coils and a set of frame coils separated by a coil holder from the line coils and a yoke ring, which surrounds the frame coils. The first and second influencing means are positioned at some distance from each other. The first influencing means is positioned close to the electron gun and is referred to as the xe2x80x9cgun quadrupolexe2x80x9d, whereas the second influencing means comprises a coil for generating a magnetic quadrupole field, which is wound around the yoke ring and is referred to as the xe2x80x9cyoke quadrupolexe2x80x9d.
It has been observed that during operation a pattern of alternating lighter and darker vertical bars on one side of the screen is visible. This pattern is attributed to ringing of the yoke quadrupole. Ringing is a phenomenon caused by high-frequency electromagnetic oscillations in coils. Ringing may also occur in other coils that influence the deflection of the electron beam.
It is an object of the invention to suppress ringing caused by coils that influence the deflection of the electron beam. The cathode ray tube according to the invention is characterized in that the coil comprises an electrically conductive coil holder and coil windings, having an electrically conductive outer layer. As a result the electrically conductive layer of the coil wires and the coil holder cooperate with each other so as to form a conductive path between successive coil windings.
The yoke quadrupole comprises many wire packages that are partly or completely separated from each other. Further, the wires are wound not very close together within each package and even tend to spread out, i.e. within a package the wires hardly touch each other. The conductive path between the coil windings provides a damping mechanism for the ringing phenomenon, by which it is completely suppressed.
The use of an electrically conductive coil holder as such in order to suppress ringing is known from JP 09320488. The holder that has a relatively high impedance is used in a deflection unit that comprises a set of line and frame deflection coils and is made from a mould material in which carbon is mixed. This type of ringing, known as common-mode ringing, can be explained as follows. In the deflection unit, the line coils are positioned close to the frame coils. At relatively high frequencies a displacement current can pass through a parasitic capacitance that exists between the line and frame coils. In that manner, a common-mode current flows in both line and frame coils and can return through drive electronics. By making the coil holder weakly electrically conductive a shield structure is created that prevents the common-mode current to flow from the line coils to the frame coils. It is to be noted that even in the presence of such a shield structure ringing of individual coils can still occur, in that case a so-called differential mode current flows from one terminal of the coil (through the coil) to a second terminal of the coil. Applying a shield structure therefore does not prevent differential mode ringing, which is the type of ringing occurring e.g. in the yoke quadrupole coil.
The use as such of windings having an electrically conductive outer layer, preferably by using so-called ringing free wire, in line and frame coils is known from WO 99/66526. The line deflection coil system as well as the frame deflection coil system can be regarded as a resonance circuit with a natural frequency. The impedance of the circuit is frequency-dependent and exhibits a maximum at or around the natural frequency of the resonance circuit. The steeper the slope of the resonance characteristic, the more ringing occurs. A measure of the slope of the resonance characteristic is the width of the resonance peak. The application of the conductive layer increases the width of the resonance peak, so that resonances in the line deflection coil system are damped more rapidly, which reduces ringing. For effective ringing suppression it is important that the coils are closely packed, thus enabling a close contact between the conductive outer layer of the wire of the various coil windings. This is achieved in the line and frame coils of the mussel-type during the coil manufacturing process when pressing and backing steps are applied to the coils.
In the case of the yoke quadrupole coil, the use of ringing free wire alone does not prevent ringing. The windings of this coil are not closely packed as no compacting process has taken place, and the coil windings do not make sufficient electrical contact with each other.
The invention effectively suppresses ringing in coils wherein the wires are not closely packed, e.g. in the case of the yoke quadrupole coil. The conductive coil holder provides a conductive path between windings of different packages of the coil. The coil holder also improves the electrical contact between windings within a package, i.e. the windings that are in contact with the coil holder. It should be noted that in this case, the coil holder does not act as a shield, but is required for providing an electrical path. It is only in the cooperation between the coil holder and the ringing free wire that the prevention against ringing occurs.
This aspect as well as other aspects of the invention are defined by the independent claims.
Advantageous embodiments of the invention are defined by the dependent claims.
These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter.